This invention relates to Josephson logic circuits, and particularly to a DC-powered Josephson integrated circuit.
The structure of a typical conventional DC-powered Josephson logic circuit is described in detail by A.F. Hebard, S.S. Pei, L.N. Dunkleberger, and T.A. Fulton in a paper titled "A DC Powered Josephson Flip-Flop" IEEE Trans. on Magnetics, Vol. MAG-15, pp. 408-411, Jan., 1979.
FIG. 21 shows the structure of the circuit disclosed in the paper by A.F. Hebard and others. FIG. 21, shows two magnetic flux coupling type Josephson elements 2101 and 2102 which are connected in series. These elements are set to such operating conditions that when one of the elements is switched from a superconducting state to a voltage state, the other element is switched reversely from the voltage state to the superconducting state as a reaction to the former switching action, thereby realizing the DC-powered operation. The DC-powered circuit of this arrangement is called a "huffle circuit". In addition, as illustrated, these two Josephson elements are each provided with a plurality of control input lines 2103, and the threshold value logic functions of both elements to each input are utilized to realize a DC-powered OR gate or AND gate.
Two magnetic flux coupling type Josephson elements according to the A.F. Hebard system can make an OR gate or an AND gate. On the other hand, when an exclusive OR gate is realized by a combination of OR gates and AND gates, two OR gates and one AND gate or one OR gate and two AND gates are necessary. Therefore, 6 Josephson elements in total are necessary for an exclusive OR gate, and 2 switching stages are constructed from the input to the output. For high-density integration and high-speed operation of the circuits, it is of course desired to reduce the number of these elements and switching stages.
On the other hand, from the output amplitude point of view, it is possible to derive a current amplitude of several tens of microampere (.mu.A) to several milliampere (mA) from a load inductance by adjusting the threshold characteristics of the Josephson elements. However, the output voltage depends on the electrode material of the Josephson junction. When the Josephson junction is made of a typical niobium-based material, the output voltage is limited to about 2 mV across a load resistor 2106 or 2107. Thus, there is the problem that the direct interface to a semiconductor circuit is difficult.
Two series circuits, each of a plurality of Josephson elements, are connected in parallel and an amplified voltage is derived from the opposite ends of the parallel circuit as described in a paper titled "Josephson IC-Semiconductor IC Interface Circuit" written by Hideo Suzuki, Atuki Inoue, Takeshi Imamura, Nobuya Hasuo and others, in All Japan Spring Meeting of the Institute of Electronics and Communication Engineers of Japan (1989), No. SC-3-8, pp. 5-357 to 358. FIG. 22 shows this circuit arrangement, in which a series output voltage of about 150 mV can be derived from the output end of 52 series-connected Josephson junctions 2201. However, this circuit is driven by an AC power supply, and a method for deriving a high voltage from a DC-powered circuit is not known yet.
Thus, there are still problems in high-density integration and high-speed operation of Josephson elements and in the interface of such elements to semiconductor circuits.